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Overwintering of Uncinula Necator in Austrian Vineyards

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Overwintering of Uncinula Necator in Austrian Vineyards Vitis 37 (4), 193-194 (1998) Research Note investigations, periodical surveys were carried out in nu­ merous vineyards (from bud break onwards) in order to iden­ tify primary infections with U necator. Overwintering of Uncinula necator in Austrian 0 c c u r r e n c e o f c I e i s t o t h e c i a : Canes with vineyards 10 nodes up from the basal node were collected between February and April and cut into 10 pieces consisting of one SIEGRIO STEINKELLNER node and the following intemode. Cleistothecia were counted under a stereomicroscope. Cleistothecia from bark were harvested in March 1997 S u m m a r y : Studies on the overwintering of Uncinula and 1998 by brushing them off the upper trunks and cor­ necator in Austrian vineyards were carried out from 1994 to 1998. dons of 5 vines in each vineyard. The samples were shaken There was no evidence of mycelium overwintering in infected buds vigorously in 1000 ml water and the resultant suspension and only a negligible number of cleistothecia were found over­ poured through 300 f.lm and 105 f.lm sieves which were exam­ wintering on hark. They were, however, discovered on canes in all the vineyards examined. The density of cleistothecia on these canes ined under the microscope; the number of cleistothecia was was closely related to the fungicide treatment applied during the recorded. preceeding season and there were marked annual differences in density, which peaked in 1994 and 1995. The middle part of the Results and Discussion: D e v e I o p m e n t o f f I a g canes was more frequently contaminated with cleistothecia than s h o o t s : Neither size and spread ofpatches arising from the basal or upper parts. U. necator cleistothecia are more frequent shoot infections (< 10 to >50%) nor the minimum winter in Austrian vineyards than hitherto supposed and might be a source temperature (between -5 and -14 °C) had any influence on of primary inoculum for grapevine powdery mildew infections. the growth of shoots under laboratory conditions. 84-98 % of the buds hurst and developed shoots. Only one of these K e y w o r d s : Uncinula necator, grapevine powdery mildew, cleistothecia, flag shoot, overwintering. shoots deve1oped a U necator infection which appeared as a small spot on the sixth expanded leaf and was therefore not considered to be a typical flag shoot. Furthermore, Introduction: Uncinula necator (Schwein.) Burr, the perennating bud infection was not observed in any of the causal agent of grapevine powdery mildew, was widespread vineyard surveys. The size and spread of patches resulting in most Austrian grape-growing regions in the early 1990s from shoot infection by grapevine powdery mildew may be (much more so than had been the case for many years ). This taken as an indicator of the extent of infestation of the previ­ prevalence was due to both weather conditions favouring ous year and could be considered an inauspicious portent the spread of the disease and control problems. Accord­ for the development offlag shoots, particularly in 1994 and ingly, there has been interest in Austria in the overwintering 1995. Nevertheless, flag shoots never developed in the labo­ of the fungus. As in most northern European viticultural ratory studies, as has been reported by VAN DER SPUY (1977), areas, this overwintering has been attributed to mycelium SALL and WRYSINSKY (1982) and H!LL (1997). Moreover, ex­ surviving in dormant buds. Recently, cleistothecia have also tensive vineyard surveys also ruled out the possibility that been identified as a principal source of inoculum for some U necator was overwintering as mycelium in dormant buds. northern European areas (PEZET and BoLAY 1992; KAsT 1994; As shown by HILL(l990), cold wintertemperatures (<-12 °C) HILL 1997) and this new possibility has been subject to in­ appear to kill infected buds already stressed by grapevine tense discussion in Austria. powdery mildew. In the present investigation, cuttings origi­ The objective of the research presented here was to nating from vineyards exposed to very low temperatures did investigate the overwintering of U necator in Austrian vine­ not show evident differences in shoot development and yards, and to estimate the extent to which cleistothecia are a powdery mildew infection. Furthermore, at alllocations ex­ possible source of initial inoculum for grapevine powdery amined, powdery mildew infections developed comparatively mildew infections under Austrian grape-growing conditions. late, at the onset offlowering or later. However, buds at the basal insertions are formed before flowering and pass to a Materialandmethods: Development offlag stage of dormancy in July (PRATT 1979; KoussA et al. 1994). s h o o t s : Canes that had developed from highly-infected D e n s i t y o f c I e i s t o t h e c i a o n c a n e s : The vines (with varying degrees ofinfection, as identified from occurrence of U necator cleistothecia on canes was de­ the size and spread ofinfected patches) were collected from pendent on the fungicide treatment applied during the January to April1994-1996 and placed in water for about preceeding season (Tab. 1). Canes from plots treated with 24 h to ensure consistently good watering. Subsequently, triadimefon had a significantly higher density of cleistothecia cuttings with two buds were taken, with the lower bud being than canes from plots treated with pyrifenox and 0.2 % sul­ removed. The cuttings were then cu1tivatcd on well-soaked phur. Cleistothecia density dec1ined considerab1y over the substratum (Leca GmbH, Fehring, Austria) in a growth cham­ period of the study ( 1994-1998). The significantly highest ber for 60 d at 24-28 °C, with a relative humidity of 60-95 % density of cleistothecia was observed in 1994, with an aver­ and a daylight period of 16 h. In addition to the laboratory age number of nearly 4 70 cleistothecia per cane. In 1996, 1997 and 1998 there was a sharp drop in cleistothecia den­ Correspondence to: Dr. SIEGRIO SrEINKELLNER, Institute of Plant sity. Protection, University of Agricultural Sciences, Peter-Jordan­ The greatest number of cleistothecia on a single cane Strasse 82, A-1190 Vienna, Austria. was found at Türkenschanze in 1994 (see Tab. 2), 194 SIEGRIO STEINKELLNER Tab I e 1 Table 2 Density of c1eistothecia of Uncinula necator on canes (I 0 nodes Maximum density of Uncinula necator cleistothecia in canes at per cane) of Grüner Veltliner at Türkenschanze 1994-1998 four locations in spring, 1994-1998 Treatment Mean density of cleistothecia Year Türken- Mitter- Pisen- Holz- 1994 1995 1996 1997 1998 schanze berg kopf gasse Triadimefon 469.2 70.4 1.2 0.2 0.4 1994 948 19 178 *l Pyrifenox + 0.2% sulfur 54.0 1.1 0.0 0.0 0.0 1995 286 44 *l *l 1996 3 *) 13 254 Mean 261.6c 35.8b 0.6a 0.1 a 0.2a 1997 2 1 40 F-value years 160.71 *** 1998 33 8 6 F-va1ue treatments 70.44 *** F-value years x treatments 9.87 *** *>no survey carried out. Means followed by different letters are significantly different. perennation of the fungus. However, given the negligible numher of cleistothecia found to be overwintering on bark, their importance as a starting point for grapevine powdery with 948 cleistothecia, though the density at this site sub­ mildew infections should not he overestimated. sequently declined to only one cleistothecium per cane in 1997 and 1998. Densities at Mitterberg and Pisenkopfreached The author is grateful to Ao. Prof. H. REDL (Institute ofPlant a minimum in 1997 and rose only slightly in 1998 ( although Protection, University of Agricultural Sciences, Vienna) for his cleistothecia were abundant on leaves in autumn 1997 support and many useful suggestions. ( STEINKELLNER and REoL 1998)). The densities ofcleistothecia at Holzgasse followed a similar trend to that of Türken• CoRTES!, P.; GADOURY, D. M.; SEEM, R. C.; PEARSON, R. C.; 1995: Distribu­ schanze, although the densities in 1996 and 1997 were mark­ tion and retention of cleistothecia of Uncinula necator on the edly higher compared to all other locations. lt should be bark of grapevines. Plant Dis. 79, 15-19. GADO URY, D. M.; PEARSO N, R. C.; 1988: Initiation, deve1opment, dis­ noted that large and widespread patches arising from shoot persal, and survival of cleistothecia of Uncinula necator in New infections by grapevinc powdery mildew were (in most cases) York vineyards. Phylupatho1ogy 78, 1413-1421. not inevitably accompanied hy a higher density of HILL, G. K.; 1990: The influence of annual weather patterns on epi­ cleistothecia on canes. GADOURY and PEARSON ( 1988) exam­ demics of Uncinula necator in Rheinhessen. Wein-Wiss. 45, 43- 46. 2 ined cleistothecia (at densities of 113-608·m- ) on canes in --; 1997: Oidium. Das Ding mit den Ankerchen. Dt. Weinmagazin 8, spring and noted that up to 92 % had died during winter and 24-27. failed to release ascospores. The density of cleistothecia on KAST, W. K.; 1994: Oidiumprobleme. Ursachen - Konsequenzen. Rebe 2 Wein 47, 380-383. canes at Türkenschanze ( 10 canes·m- ) reached up to KoussA, T.; BROQUEDIS, M.; BoUDARD, J.; 1994: Importance de l'acide 2 4692·m· in 1994. Even atjust 8% viahi1ity, this would mean abscissique dans Je developpement des bourgeons 1atents de vigne that cleistothecia could still he an important source of pri­ (Vitis vinifera L. var. Merlot) et plus particulil~rement dans Ia mary inoculum. Furtherattention should he paid to the fact phase de levee de dormance. Vitis 33, 63-67. that insertions from the middle part of canes were more MAGAREY , P.
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